As a system to transfer a toner image on an electrophotographic photoreceptor (hereinafter, also denoted simply as a photoreceptor) onto a recording material, there have been known an image forming system using an intermediate transfer member. In such system, one additional transfer step is introduced to the steps of transferring a toner image from an electrophotographic photoreceptor to a recording material, in which the toner image is transferred primarily from an electrophotographic photoreceptor to an intermediate transfer member and then, the primary transfer image on the intermediate transfer member is secondarily transferred to the recording material. This system has been often employed as a multiple transfer system in a so-called full-color image forming apparatus to reproduce a color-separated original image through subtractive color mixing by use of black, cyan, magenta and yellow toners.
However, a multiple transfer system employing such an intermediate transfer member, in which two separate transfers, a primary transfer and a secondary transfer are introduced and four color toner images are superimposed, often causes image defects accompanied with transfer troubles of the toner image.
In general, with regard to toner transfer troubles, there is known subjecting the toner surface to a surface treatment with an external additive such as silica, whereby enhanced transfer efficiency is achieved. However, such silica separates from the toner particle surface or is buried in the interior of the toner particle due to stress subjected from a stirring member in a developing device, stress subjected from a controlling blade to form a toner layer on a developing roller, stress subjected between a photoreceptor and a developing roller and the like, producing problems such that satisfactory transfer efficiency cannot be achieved.
To achieve enhanced secondary transfer efficiency of an intermediate transfer member, there was proposed a technique in which the surface of an intermediate transfer member is covered with a thin layer of an inorganic compound (for example, silicon oxide, aluminum oxide and the like), resulting in enhanced releasability of the toner image and achieving enhanced transfer efficiency onto recording paper or the like (as described in, for example, Patent documents 1 and 2).
Patent documents 1 and 2 describe that a layer of silicon oxide or aluminum oxide is formed on the surface of an intermediate transfer member to achieve enhanced releasability of toner particles from the intermediate transfer member. However, when an intermediate transfer member prepared in this method was subjected to an endurance test in a practical image forming apparatus, there was a problem that an oxide layer was stripped off from the surface layer upon repeated bending action and there was also a problem that a large-scaled facility such as a vacuum equipment was required to form a silicon oxide or aluminum oxide layer through vacuum deposition or spattering.
In light of the foregoing problems, methods for enhancing secondary transferability of an intermediate transfer member include a technique of forming a thin layer of an inorganic compound (such as silicon oxide or aluminum oxide) through an atmospheric plasma treatment process. When forming a thin an inorganic compound layer in this method, there may be employed a constitution comprised of a first layer (adhesive layer) and a second layer (surface layer or hard layer) to achieve superior durability (for example, adhesion of a thin-layer to a substrate or hardness of a surface thin-layer). In such layer constitution, to incorporate a stress relaxation function to the first layer, the reaction condition of an auxiliary gas (for example, nitrogen gas, hydrogen gas and the like) is controlled so that thin layer formation is not completed with the raw material, whereby the carbon content within the formed thin-layer is controlled to remain constant.
There was also proposed a method of forming an inorganic coating layer on the surface of an intermediate transfer member (as described in, for example, patent document 3). In the method proposed in the patent document 3, it was proved that an increased amount of colloidal silica added to an inorganic coating layer resulted in enhanced releasability of the toner, leading to improved transfer efficiency, however, repeated bending action in a durability test caused cracking of the inorganic coating layer, rendering addition of more colloidal silica than a given amount to be infeasible. Accordingly, there were problems that sufficient releasability was not achieved and a transfer efficiency was not enhanced more than a given level.    Patent document 1: Japanese Patent Application Publication JP 9-212004A,    Patent document 2: JP 2001-347593A    Patent document 3: JP 2000-206801